Many fluids are not rheologically simple, that is to say they have properties which cannot be described by a single figure for viscosity. The present invention is particularly applicable to the measurement of viscosity of this class of materials.
Materials which possess viscoelasticity can be described by the complex rigidity function EQU G*=G'+j G"
where G' and G" represent the parts of the complex rigidity modulus which are respectively in phase and in quadrature with the applied stress. Both G' and G" are functions of frequency.